Download Mitosis and Cytokinesis

Mitosis and Cytokinesis
Jessica Harwood
Douglas Wilkin, Ph.D.
Say Thanks to the Authors
Click http://www.ck12.org/saythanks
(No sign in required)
To access a customizable version of this book, as well as other
interactive content, visit www.ck12.org
AUTHORS
Jessica Harwood
Douglas Wilkin, Ph.D.
EDITOR
Douglas Wilkin, Ph.D.
CK-12 Foundation is a non-profit organization with a mission to
reduce the cost of textbook materials for the K-12 market both
in the U.S. and worldwide. Using an open-content, web-based
collaborative model termed the FlexBook®, CK-12 intends to
pioneer the generation and distribution of high-quality educational
content that will serve both as core text as well as provide an
adaptive environment for learning, powered through the FlexBook
Platform®.
Copyright © 2015 CK-12 Foundation, www.ck12.org
The names “CK-12” and “CK12” and associated logos and the
terms “FlexBook®” and “FlexBook Platform®” (collectively
“CK-12 Marks”) are trademarks and service marks of CK-12
Foundation and are protected by federal, state, and international
laws.
Any form of reproduction of this book in any format or medium,
in whole or in sections must include the referral attribution link
http://www.ck12.org/saythanks (placed in a visible location) in
addition to the following terms.
Except as otherwise noted, all CK-12 Content (including CK-12
Curriculum Material) is made available to Users in accordance
with the Creative Commons Attribution-Non-Commercial 3.0
Unported (CC BY-NC 3.0) License (http://creativecommons.org/
licenses/by-nc/3.0/), as amended and updated by Creative Commons from time to time (the “CC License”), which is incorporated
herein by this reference.
Complete terms can be found at http://www.ck12.org/terms.
Printed: January 15, 2015
CONTRIBUTORS
Doris Kraus, Ph.D.
Niamh Gray-Wilson
Jean Brainard, Ph.D.
Sarah Johnson
Jane Willan
Corliss Karasov
www.ck12.org
C HAPTER
•
•
•
•
•
Chapter 1. Mitosis and Cytokinesis
1
Mitosis and Cytokinesis
Explain the importance of mitosis.
Explain chromosome structure.
Define sister chromatids.
List the phases of mitosis.
Summarize the phases of mitosis.
How is your DNA organized?
Your DNA is organized into chromosomes, the pink structures pictured above. Your DNA doesn’t always look so
pretty, though. It only winds tightly into chromosomes when the cell is getting ready to divide. If your DNA wasn’t
organized into chromosomes, your DNA would look like a mass of strings and would be difficult to divide up!
Mitosis and Chromosomes
The genetic information of the cell, or DNA, is stored in the nucleus. During mitosis, two nuclei (plural for nucleus)
must form, so that one nucleus can be in each of the new cells after the cell divides. In order to create two genetically
identical nuclei, DNA inside of the nucleus must be copied or replicated. This occurs during the S phase of the cell
cycle. During mitosis, the copied DNA is divided into two complete sets, so that after cytokinesis, each cell has a
complete set of genetic instructions.
Chromosomes
To begin mitosis, the DNA in the nucleus wraps around proteins to form chromosomes. Each organism has a unique
number of chromosomes. In human cells, our DNA is divided up into 23 pairs of chromosomes. Replicated DNA
forms a chromosome made from two identical sister chromatids, forming an "X" shaped molecule ( Figure 1.1).
The two chromatids are held together on the chromosome by the centromere. The centromere is also where spindle
fiber microtubules attach during mitosis. The spindles separate sister chromatids from each other.
1
www.ck12.org
FIGURE 1.1
The DNA double helix wraps around proteins (2) and tightly coils a number of
times to form a chromosome (5). This
figure shows the complexity of the coiling
process. The red dot shows the location
of the centromere, which holds the sister chromatids together and is where the
spindle microtubules attach during mitosis
and meiosis. Notice that a chromosome
resembles an "X."
Four Phases of Mitosis
During mitosis, the two sister chromatids must be divided. This is a precise process that has four individual phases
to it. After the sister chromatids separate, each separate chromatid is now known as a chromosome. Each resulting
chromosome is made of DNA from just one chromatid. So: each chromosome after this separation is made of "1/2
of the X." Through this process, each daughter cell receives one copy of each chromosome. The four phases of
mitosis are prophase, metaphase, anaphase and telophase ( Figure 1.2).
1. Prophase: The chromosomes "condense," or become so tightly wound that you can see them under a microscope. The membrane around the nucleus, called the nuclear envelope, disappears. Spindles also form and
attach to chromosomes to help them move.
2. Metaphase: The chromosomes line up in the center, or the equator, of the cell. The chromosomes line up in
a row, one on top of the next.
3. Anaphase: The two sister chromatids of each chromosome separate as the spindles pull the chromatids apart,
resulting in two sets of identical chromosomes.
4. Telophase: The spindle dissolves and nuclear envelopes form around the chromosomes in both cells.
FIGURE 1.2
An overview of the cell cycle and mitosis: during prophase the chromosomes
condense, during metaphase the chromosomes line up, during anaphase the sister
chromatids are pulled to opposite sides of
the cell, and during telophase the nuclear
envelope forms.
2
www.ck12.org
Chapter 1. Mitosis and Cytokinesis
After telophase, each new nucleus contains the exact same number and type of chromosomes as the original cell.
The cell is now ready for cytokinesis, which literally means "cell movement." During cytokinesis, the cytoplasm
divides and the parent cell separates, producing two genetically identical cells, each with its own nucleus. A new
cell membrane forms and in plant cells, a cell wall forms as well. Below is a representation of dividing plant cells (
Figure 1.3).
FIGURE 1.3
This is a representation of dividing plant
cells. Cell division in plant cells differs
slightly from animal cells as a cell wall
must form. Note that most of the cells are
in interphase. Can you find examples of
the different stages of mitosis?
Summary
• The DNA in the nucleus wraps around proteins to form chromosomes.
• During mitosis, the newly duplicated chromosomes are divided into two daughter nuclei.
• Mitosis occurs in four phases, called prophase, metaphase, anaphase, and telophase.
Explore More
Use the resource below to answer the questions that follow.
• Mitosis by NDSU VCell Productions at http://www.youtube.com/watch?v=C6hn3sA0ip0 (6:10)
MEDIA
Click image to the left or use the URL below.
URL: http://www.ck12.org/flx/render/embeddedobject/57346
1. When does the "classic" chromosome structure of DNA appear during mitosis?
2. What problems do you think might arise if the chromosomes did not align during metaphase?
3. When do the nuclear envelopes reform? What problems might arise if a cell started forming the nuclear
envelopes earlier?
3
www.ck12.org
4. In what stage do cells spend most of their "life"?
5. How long does mitosis take in the typical eukaryotic cell?
Review
1.
2.
3.
4.
5.
What are chromosomes?
What are the four phases of mitosis, in the correct order?
In what phase of mitosis are chromosomes moving toward opposite sides of the cell?
Compare the two nuclei that form as a result of mitosis?
What is cytokinesis, and when does it occur?
References
1. Magnus Manske. How DNA coils to make a chromosome . Public Domain
2. Zachary Wilson and Mariana Ruiz Villarreal (LadyofHats), using images by LadyofHats (http://commons.wikimedia.org/w
An overview of the cell cycle and mitosis . CC BY-NC 3.0
3. Edmund Beecher Wilson. Drawing of dividing plant cells . Public Domain
4